Photocurrents and photovoltages produced by a photodetector, composed for example of amorphous silicon, as disclosed in U.S. Pat. No. 4,064,521, incorporated herein by reference, can be significantly reduced by electrical shorts or shunts formed during the fabrication of the detector. Electrical shorts occur either when there is an opening in the semiconductor material which causes the front and back electrodes to contact one another or if there is a conductive metal path extending through the semiconductor material. A shunt is the loss of charge in the semiconductor material due either to an imperfect rectifying barrier or to the formation of an ohmic contact via a high-work function metal rather than a Schottky rectifying barrier. The probability of the occurrence of a defect will increase rapidly with increasing detector area.
In order to economically fabricate large-area photodetectors, methods must be developed to either eliminate such shorts or shunts during fabrication or after fabrication. Elimination of defects during fabrication requires facilities and processing conditions which will greatly increase the cost of the photodetector. For this reason it is desirable to develop methods for the removal of such defects after the fabrication of the detector has been completed.
The process for removal of a defect in a photodetector comprising a material such as gallium arsenide, cadmium telluride, single-crystal or amorphous silicon, requires the etching out of the defect. Attempts made to remove the defects by the application of a reverse-bias voltage have tended in the past to increase the incidence of shorts and shunts in the photodetector. Nostrand et al in U.S. Pat. No. 4,166,918 have disclosed that defects in the solar cell containing a cermet layer between the active region and the exposed electrode can be reduced by applying a reverse-bias voltage to the cell which is sufficient to burn out electrical shorts and shunts but less than the breakdown voltage of the photodetector. This approach is useful for a defect of intermediate size but fails for small and very large defects where the heat generated is either too small or spread over too large an area to burn out the defect. Swartz in U.S. Pat. No. 4,385,971 has disclosed that defects in a solar cell can be electrochemically eliminated by connecting the device as one electrode to one polarity and a second separate electrode to the opposite polarity of a dc power supply and immersing these electrodes in an anodizing chemical etching electrolyte. Application of a reverse-bias voltage between the electrodes causes either electrochemical etching or mechanical removal of the defects.
Other techniques for the removal of defects have included placing a cell having such defects under a reverse-bias voltage and locating defects using either thermally sensitive liquid-crystal techniques or infrared-imaging techniques. After the defect is located, the bias is removed and an etchant is locally applied to the exposed electrode of the cell thereby removing the electrode locally and eliminating the electrical contact to the defect. This approach has the undesirable feature that the etching process is not locally selective and, as a result, an unnecessarily large portion of the exposed electrode is removed due to the finite size of the acid droplet and the operator's inability to determine when the etching process has proceeded far enough to eliminate the defect.